... topless, gathering being pretentious (4)
I believe the answer is:
arty
'pretentious' is the definition.
(similar in meaning)
'topless gathering' is the wordplay.
'topless' means to remove the first letter.
'gathering' becomes 'party' (party is a kind of gathering).
'party' with its initial letter removed is 'ARTY'.
'being' acts as a link.
(Other definitions for arty that I've seen before include "Like an aesthete" , "Pretentiously creative" , "Having pretensions about paintings, music, etc" , "Culturally creative" , "aspiring to be creative?" .)